This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks are tailored for high-speed and high-throughput user groups. With a growing number of subscribers and increasing demand for bandwidth, traditional macro base stations can barely meet subscribers' requirements. Especially in some hotspots, simple macro cell coverage is not sufficient for meeting traffic requirements. Such homogenous coverage inevitably causes blind spots that may impact user experience.
With introduction of small cells, such as picocells or femtocells, and relay nodes, LTE network topology becomes more flexible and may handle blind spots. This network topology is defined as heterogeneous network (HetNet) in 3GPP Release 10. In a HetNet, low power nodes (LPNs), such as remote radio units/heads (RRUs/RRHs), pico eNodeBs, home eNodeBs, and relay nodes, which may form small cells, are deployed in a regular macro cell.
However, in such a HetNet, when a same carrier frequency is applied for the sake of spectrum efficiency, inter-cell interference between macro and small cells turns out to be severe and has different characteristics from a traditional homogenous network with macro cells only. Strong received power from the macro cell makes the small cell coverage narrow. This causes a limited cell splitting gain. Then Cell Range Expansion (CRE) is proposed, which allows a User Equipment (UE) to be served by a cell with weaker received power. CRE is simple and typical one of alternative cell associations to enhance offloading but it causes downlink (DL) interference issue in not only data but also control channels.
In particular, FIG. 1 explains different interference situations when CRE is applied or not applied in a HetNet. FIG. 1 (a) exemplarily illustrates an example HetNet that comprises a macro-cell base station BS1, e.g. a macro-eNodeB that forms a macro cell C1 and a small-cell base station BS2, e.g. a pico-eNodeB that forms a small cell C2, wherein CRE is not applied. UE1 is located close to the cell edge of the small cell C2 and outside the small cell C2, while UE2 is also located close to the cell edge of the small cell C2 but within the small cell C2. In this case, UE1 has a much longer distance away from BS1 than from BS2. Thus, UE1's uplink (UL) transmission, which is targeted to satisfy the reception sensitivity at BS1, forms strong interference to the small cell. This interference to the small cell is defined as UL interference. The interference from DL transmission in the macro cell to the small cell is defined as DL interference. In this situation as illustrated in FIG. 1(a), the UL interference from the macro cell causes a more serious impact on the small cell than the DL interference from the macro cell does. Therefore, this type of interference is defined as “UL-dominant interference” herein.
FIG. 1 (b) illustrates another example HetNet, which differs from the HetNet of FIG. 1(a) only in that CRE is applied. With the CRE, the small cell C2 of BS2 is expanded to be an expanded small cell C3. UE1 is located close to the cell edge of the expanded small cell C3 and outside the small cell C3, while UE2 is also located close to the cell edge of the small cell C3 but within the small cell C3. Due to the expansion of the small cell, the distance between UE1 and BS1 is reduced relative to the distance in FIG. 1(a). As a result, compared to the interference situation of FIG. 1(a), the UL interference caused by UE1 to the small cell is reduced. However, the DL interference from the macro-cell to UE2 is much stronger. In this case, the DL interference from the macro cell causes a more serious impact on the small cell than the UL interference from the macro cell does. Therefore, this type of interference is defined as “DL-dominant interference” herein.
With or without CRE application, the inter-cell interference situation in a HetNet turns out to be UL interference dominant or DL interference dominant. To handle such interference, protected subframes may be configured to form a timing pattern in the subframe configuration.
For example, to make UEs, e.g. UE2, in the small cell survive with the strong UL interference from the macro cell as illustrated in FIG. 1(a) without CRE being applied, a subset of UL subframes may be allocated as protected subframes for them. On these protected subframes, the UEs in the macro cell that are close to the cell edge of the small cell are restricted from being scheduled. Correspondingly, to make UEs, e.g. UE2, in the small cell survive with the strong DL interference as illustrated in FIG. 1(b) with CRE being applied, a kind of almost blank subframes (ABSs) is defined in 3GPP standard, wherein the power on some physical channels and/or some activity are reduced (including no transmission), which are described in reference documents [1] and [2]. The macro-cell base station may mute Physical Downlink Shared Channel (PDSCH) for data transmission on ABSs, only remains control channels. On these ABSs, the small-cell base station may schedule UEs that are located in the extended area of the extended small cell.
In prior-art, the evolved inter-cell interference coordination (eICIC) technique including an almost blank subframe (ABS) pattern and cell range extension (CRE) is proposed to coordinate inter-cell interference between a small-cell (like pico-cell, femto-cell) and its neighbor macro-cell in a LTE/LTE-Advanced (LTE-A) HetNet. Such ABS pattern configuration may be embodied as the allocation of protected subframes for the small-cell. However, how to allocate the protected subframes for both sides constitutes a primary issue in this technique. In a Frequency Division Duplex (FDD) system, the allocation of protected subframes may be arbitrary, because a DL subframe is one-to-one paired with a UL subframe with regard to the scheduling grant, data transmission, and ACK/NACK feedback.
However, in an LTE Time Division Duplex (TDD) HetNet system, TDD UL-DL configurations as defined in 3GPP TS 36.211 are used. In most of these UL-DL configurations, there are asymmetric numbers of DL and UL subframes, some of which are involved in DL or UL Hybrid Automatic Repeat Request (HARQ) processes, while others are not. If the protected subframes are allocated arbitrarily or unreasonably, there could be a risk of link failure in either macro cell or small cell.